Which contraction term describes the phase where the muscle shortens to generate force?

• A. Isotonic contraction
• B. Concentric
• C. Eccentric
• D. Isometric

Answer: (B)

The contraction phase in which a muscle actively shortens to produce movement is called concentric contraction. During this phase, the muscle fibers shorten as cross-bridges form between actin and myosin and slide past each other, generating force to lift or move a load. A common example is lifting a dumbbell in a bicep curl—the biceps shorten as you raise the weight, producing the movement.

This differs from isometric contraction, where the muscle tenses but doesn’t change length, and from eccentric contraction, where the muscle lengthens while producing force—as when you slowly lower that dumbbell. Isotonic contractions describe dynamic movements where the muscle changes length under a constant load and include both concentric and eccentric phases, but the specific shortening phase is best described by concentric contraction.